What is the if-then-else structure?

Answer 1

This is one of the most simple flow control mechanisms in programming. It follows very closely to the spoken-language version.

IF something is true

THEN do something

ELSE do something else

if x == 1

print "x is one!"

else

print "x is not one!"

Answer 2

One of the statements, obviously.

Syntax:

if (expression) statement

or

if (expression) statement1

else statement2

Example1:

if (i==1) puts ("Hey, i==1");

Example2:

if (i==1) puts ("Hey, i==1");

else puts ("Oh, i<>1");

Example3:

if (i==1) puts ("Hey, i==1");

else if (i==2) puts ("Hey, i==2");

else printf ("i==%d\n", i);

Answer 3

if-else Branching

The basic format of an if statement is as follows:

if (booleanExpression) {

System.out.println("Inside if statement");

}

The expression in parentheses must evaluate to a boolean value, either a true or false. The following code demonstrates a legal if-else statement:

if (x > 3) {

System.out.println("x is greater than 3");

} else {

System.out.println("x is not greater than 3");

}

The else block is optional, so you can also use the following:

if (x > 3) {

y = 2;

}

z += 8;

a = y + x;

The preceding code will assign 2 to y if the test succeeds (i.e., x really is greater than 3), but the other two lines will execute regardless. Even the curly braces are optional if you have only one statement to execute within the body of the conditional block. The following code example is legal:

if (x > 3) // bad practice, but can be seen on the exam

y = 2;

z += 8;

a = y + x;

Sun considers it good practice to enclose blocks within curly braces, even if there's only one statement in the block. Be careful with code like the above, because you might think it should read as

"If x is greater than 3, then set y to 2, z to z + 8, and a to y + x."

But the last two lines are going to execute no matter what! They aren't part of the conditional flow. You might find it even more misleading if the code were indented as follows:

if (x > 3)

y = 2;

z += 8;

a = y + x;

You might have a need to nest if-else statements. You can set up an if-else statement to test for multiple conditions. The following example uses two conditions so that if the first test fails, we want to perform a second test before deciding what to do:

if (price < 300) {

buyItNow();

} else {

if (price < 400) {

getApprovalFromWife();

}

else {

dontBuyItNow();

}

}

This brings up the other if-else construct, the if, else if, else. The preceding code could be rewritten:

if (price < 300) {

buyItNow();

} else if (price < 400) {

getApprovalFromWife();

} else {

dontBuyItNow();

}

There are a couple of rules for using else and else if:

• You can have zero or one else for a given if, and it must come after any else ifs.

• You can have zero to many else ifs for a given if and they must come before the else.

• Once an else if succeeds, none of the remaining else ifs or elses will be executed.